In a high pressure polymerization of ethylene, hexane is commonly employed as a solvent or carrier for catalysts and other minor components fed to the polymerization reactor. Hexane is used for this purpose not only in the homopolymerization of ethylene, but also in the copolymerization of ethylene with other monomers such as vinyl acetate and alkyl acrylates.
The hexane employed for this purpose is of a highly purified grade so that it contains no components having an adverse effect upon the polymerization. For this reason, the hexane contained in the discharge or recycle stream from the polymerization reactor is recovered for reuse. The recovery of such hexane normally is not difficult, when the ethylene polymerization plant is employed solely to prepare ethylene homopolymers.
In a typical ethylene polymerization plant, several polymerization reactors are provided and the discharge or recycle streams of the several reactors usually are combined for purification and recycling. The combined discharge stream for recycling will contain unpolymerized comonomers such as vinyl acetate and alkyl acrylates, when copolymers of such monomers are being produced in one or more of the polymerization reactors. Very substantial difficulties are encountered in recovering hexane from such combined discharge streams by reason of the fact that hexane forms azeotropes with both vinyl acetate and alkyl acrylates. It is, of course, essential to recover the hexane in a highly purified form if it subsequently is to be employed as a carrier for components to be employed in the homopolymerization of ethylene. The problem of recovering purified hexane and the concomitant problem of avoiding the introduction of vinyl acetate and/or alkyl acrylates in the intended homopolymerization of ethylene is of such magnitude that frequently operators of ethylene polymerization plants will burn such combined discharge streams as fuel in lieu of recovering the hexane therefrom.